It does have its risks - you basically un-hide root-/system-files and folders and gain access to all sorts of otherwise hidden settings.
It allows you to use ceratin apps that don't work on un-rooted phones - such as the shootMe screenshot apps, extented file-explorers; apps managers that allow you to uninstall apps that came pre-installed by Samsung, but that you may never want to use...
Plus, rooting is a prerequisite for modifying your Galaxy with custom-roms.
If any of this sounds like gobble di gook, you should probably not root. Although: the Galaxy S is extremely easy to root (and un-root if you don't like what you see...)...